A hard disk drive typically includes one or more spinning disks stacked above each other on a spindle, a disk drive controller, a rotary actuator and an actuator retract circuit. These elements typically reside in a chassis or housing and are supplied with external cable connectors.
The rotary actuator consists of an arm equipped with a head for reading and writing data in generally radial and concentric tracks in the recording layers of the individual disk. The actuator is usually driven by an attached voice coil motor (VCM). Cables are connected with the actuator to facilitate transmitting signals to and from the head and to power the VCM. The disk drive controller is typically an electronic circuit that controls all functions of the hard disk drive.
During regular operation of the drive, the controller controls the actuator motions including the movements to and from a parking position, at which parking position the actuator is placed when the drive is not under operation. However, if the power supply to the drive is shut off unexpectedly, the actuator may not be in the proper parking position. Since the controller requires an external power to operate, it can not park the actuator after unexpected power supply shut-off. Therefore, an independent retract circuit parks the actuator in such cases. Such a retract circuit has to be able to power and control the retraction or withdrawal of the actuator from proximate the disk surface into the parking position within a critical time period during which the spinning disks slow down to a minimal rotational speed. The minimal rotation speed guarantees sufficient supporting air flow between the disk surface and an air bearing surface of the read and write heads to keep them at a minimum flying height. In case the supporting air flow should fall beneath a critical value, the heads are likely to crash and damage the disk surface. Moreover, if the heads were to come to rest on the smooth disk surface, they may adhere to the disk through a process known as stiction.
One method of parking the drive head during a power-off situation is to move it to the parking ramp, typically located inside or outside the disk access area. Each head of the actuator is mounted on a suspension, and a tab is retracted onto a wedge shaped ramp. When the tab reaches the predetermined parking position on the ramp, the actuator is held in place with either a mechanical or inertial latch.
In one common approach, the electrical energy utilized for retracting the head to a parking position is generated by a back electromotive energy generated from the kinetic energy stored in the rotating disk stack. The kinetic energy is thereby converted into a back electromotive voltage (BEMV) by utilizing the disk motor as a generator. The rectified BEMV is electronically connected across the VCM, which generates a torque on the actuator in the desired direction into the parking position.
In another common approach, the electrical energy used for retracting the actuator to the parking position is stored in a capacitor during normal operation, which energy is then released to retract the head into the parked position. One prior art solutions which use such a charge capacitor selectively connects the capacitor to the voice coil of an actuator through switches. Using this technique, the voltage and/or current is not controlled as it is delivered to the voice coil, which can be problematic in that it has difficulty to protect actuator rebound at an outer crash stop when the actuator moving velocity is to high, and that it also has difficulty to apply enough torque to get over the parking ramp.
In yet another conventional technique, the actuator is pulse modulated in an effort to extend the discharge time of the capacitor. The voice coil's self-induction energy is utilized to keep coil current, although there is not an active-controlled voltage across the voice coil. Notably, the actuator velocity is proportional to the applied voltage across the voice coil.
The prior solutions fail to both suppress the initial velocity when the actuator is over the disk, and also generate adequate torque to get the actuator over a parking ramp.